[PATCH] xenbus: avoid zero returns from read()

[Daniel De Graaf <dgdegra@tycho.nsa.gov>]

It is possible to get a zero return from read() in instances where the
queue is not empty but has no elements with data to deliver to the user.
Since a zero return from read is an error indicator, resume waiting or
return -EAGAIN (for a nonblocking fd) in this case.

Signed-off-by: Daniel De Graaf <dgdegra@tycho.nsa.gov>

---
diff --git a/drivers/xen/xenfs/xenbus.c b/drivers/xen/xenfs/xenbus.c
index 88c87c9..0ddef43 100644
--- a/drivers/xen/xenfs/xenbus.c
+++ b/drivers/xen/xenfs/xenbus.c
@@ -121,6 +121,7 @@ static ssize_t xenbus_file_read(struct file *filp,
 	int ret;
 
 	mutex_lock(&u->reply_mutex);
+again:
 	while (list_empty(&u->read_buffers)) {
 		mutex_unlock(&u->reply_mutex);
 		if (filp->f_flags & O_NONBLOCK)
@@ -159,6 +160,8 @@ static ssize_t xenbus_file_read(struct file *filp,
 					struct read_buffer, list);
 		}
 	}
+	if (i == 0)
+		goto again;
 
 out:
 	mutex_unlock(&u->reply_mutex);